1. Field of the Invention
The present invention relates generally to transistors, and more specifically to an improved radio frequency (RF)/Microwave bipolar junction transistor device layout.
2. Description of the Prior Art
The device layout of high frequency power transistor circuits can be critical to their performance. At RF and microwave frequencies, the parasitics internal and external to the device often become a problem. For example, bondwire inductance can introduce feedback between the input and output impedance matching circuits, which may be positive or negative depending on the amplifier configuration. This can be detrimental to gain and stability. As the operating frequency of the circuit increases, this problem is exacerbated.
Connection of the base lead to the transistor die has typically been accomplished by connecting bondwires from the base of the transistor to matching capacitor ground bars which form an integral part of both the input and the output matching circuit MOS capacitors. The ground bars are typically composed of a high conductivity material such as doped silicon, which results in a small but significant resistance from path to ground. The large current flowing in the output circuit generates a significant voltage drop across this resistance which is common to input and output circuits, which in turn contributes to negative feedback. This feedback voltage is subtracted from the input signal voltage at this point in the circuit; therefore, transistor power gain is limited.
In addition to the restraints placed upon the power gain by elements common to the input and output circuits, the layout of the collector circuit can adversely affect transistor performance. The collector connection is made to the backside of the transistor chip (the emitter and base connections are made to topside contacts). The transistor chip is mounted to an electrically isolated collector island that has a slightly larger dimension than the chip. The collector lead can then be connected to this "collector island" using a number of bondwires.
During the process of mounting the chip into its package, the island can become covered in silicon/gold eutectic to which a gold bondwire can not reliably bond. Aluminum wires, which often possess inferior bond pull strength when bonded to silicon/gold, must be used to bond the collector lead to the transistor die. As a practical matter, aluminum bondwires may also be undesirable if it is necessary to use a machine different from that used for gold bond wires since an extra wirebond step would be required during assembly.